Conventional refrigeration equipment in the 1-3 ton cooling class relies on positive displacement compressors as they are most efficient at low output shaft speeds. This low speed is tailored to applications such as automobiles where belt drive running sheaves in excess of 8,000 RPM are uncommon, and to commercial appliances where 60 Hz commercial power make shaft speeds in excess of 3,500 RPM impossible without brushes or gearboxes.
In order to keep this equipment as small as possible, the compressor displacement has to be kept as small as possible. This practice, in general, minimizes production costs. In concert, a small displacement compressor requires the use of a high pressure, high density refrigerant in order to pump a sufficient mass flow of refrigerant to produce the desired refrigeration effect.
These conventional refrigerant compressors contain an oil sump or reservoir as part of the compressor housing. Lubricating oil is pumped from the reservoir to the compressor's bearings. Examples of such systems can be found in U.S. Pat. Nos. 3,449,922 and 4,213,307. However, in these centrifugal compressors, the refrigerant is isolated from the lubricant. This approach requires seals between the lubrication system and the refrigeration system to isolate the oil from the refrigerant, a separate pump for circulating lubricant to moving parts and a complex mechanism to separate oil that leaks into the refrigerant.